Wireless communication has lifted the burden of wiring in wired communication of the past, and furthermore, contributed to its use as a technique for realizing communication in moving objects. As a standard relating to a wireless LAN (Local Area Network), for example, IEEE (The Institute of Electrical and Electronics Engineers) 802.11 can be exemplified. IEEE 802.11a/g has already been widely distributed.
According to the standard of IEEE 802.11a/g, a modulation scheme in which a communication rate of 54 Mbps at the maximum (physical layer data rate) is applied is supported in 2.4 GHz or 5 GHz frequency band, using Orthogonal Frequency Division Multiplexing (OFDM). In addition, in IEEE 802.11n that is the extended standard of the foregoing, an even higher bit rate is realized by adopting a MIMO (Multi-Input Multi-Out) communication scheme. Herein, the MIMO is a communication scheme in which a plurality of antenna elements is provided in both of a transmitter side and a receiver side to realize a spatially multiplexed stream (as is well known). While a high throughput (HT) over 100 Mbps has been achieved with IEEE 802.11n, an even higher speed increase has been demanded as the amount of transmitted content information increases.
For example, it is possible to enhance throughput in one-to-one communication, keeping lower-level compatibility by increasing the number of streams spatially multiplexed with the number of antennas in a MIMO communication device increased. However, in the future, the enhancement of throughput by a plurality of users as a whole will be demanded in communication, in addition to throughput per user.
In the conference of the working group of IEEE 802.11ac, it was aimed to use a frequency band of equal to or lower than 6 GHz, and to set a wireless LAN standard of which the data transmission rate is over 1 Gbps, but for the realization, Space Division Multiple Access (SDMA) scheme in which a plurality of users shares wireless resources on a space axis is effective as the multi-user MIMO (MU-MIMO), or SDMA.
Currently, the Space Division Multiple Access is under examination as one of an underlying technology of a next generation mobile telephone system based on Time Division Multiple Access (TDMA) such as a PHS (Personal Handyphone System), and LTE (Long Term Evolution). In addition, attention has been paid to one-to-multiple communication as described above in the field of wireless LAN, but there is almost no applicable example thereof. It is also considered that the reason is that multiplexing a plurality of users with satisfactory efficiency in packet communication is difficult.
For comparison, a communication system has been proposed which is attained by combining two techniques of carrier sensing and the SDMA by adaptive array antennas in the standard of IEEE 802.11 of the past, using RTS, CTS, ACK packets that have a packet format and keep lower-level compatibility with the standard of IEEE 802.11 of the past (for example, refer to PTL 1).
Herein, when the SDMA is applied to a wireless LAN, a case can be considered in which a variable-length frame is multiplexed on the same time axis. There is no problem if the lengths of transmission data for each of a plurality of users are the same as a whole, but if frame lengths multiplexed according to differences in the lengths of transmission data differ, the total transmission power radically changes as the number of frames multiplexed during transmission period increases. If frames with different lengths are multiplexed without changes and transmitted, there is a possibility of bring about a problem in various senses that power distribution in a frame for RCPI (Received Channel Power Indicator) that is standardized in IEEE 802.11 is not regular, which triggers an unstable operation in Auto Gain Control (AGC) at a reception side. For this reason, it is necessary to transmit frames multiplexed on the same time axis with the same frame length at the final stage, even if the transmission data lengths differ for each user.
In a system of a fixed frame format such as a cellular system of the past, for example, it is possible to perform frame compensation, or the like with insertion of data for diversity (for example, refer to PTL 2), scheduling of assigned time (for example, refer to PTL 3), a variable data rate (for example, refer to PTLs 4 and 5), and variable channel configuration (for example, refer to PTL 6). With regard to this, it is difficult to apply this related art as the system which employs a variable-length format such as a wireless LAN has a fundamentally different configuration.